Swift之交换/替换方法

运行环境

Xcode 10.1
Swift 4.2

背景知识

Swift 是一种强类型语言。即默认类型是安全的静态类型。纯Swift类的函数调用已经不再是OC的运行时发送消息，而是类似于C++的vtable,在编译时就确定了调用哪个函数，所以没法通过runtime获取方法，属性。Swift中的动态性可以通过OC运行时来获得，动态性最常用的就是方法替换(Method Swizzling)。

Swift中的动态修饰符

@objc: 将`Swift 函数暴露给 OC运行时，但是它仍然不能保证完全动态，编译器会尝试去对它做静态优化
@dynamic: 动态功能修饰符，它能保证函数，属性可以获得动态性

注意：继承自NSObject的Swift类，其继承自父类的方法具有动态性，因为是NSObject中的方法,其他在Swift中自定义方法，属性需要加@objc与@dynamic 必须同时加上者两个修饰符获得动态性，否则要报错"'dynamic' instance method 'sayHello' must also be '@objc'" 。若方法的参数，属性类型为Swift特有的，无法映射到OC的类型上(例如，Character,Tuple),不能添加@dynamic

定义一个需要替换方法实现的类(不继承`NSObject`)

class SayHello {
    // 需要添加dynamic修饰，否则无法使用runtime
    @objc dynamic func sayHello(_ name: String) {
        print("Hello \(name)!")
    }
}

两种方案

方案一：用Swift中的unsafeBitCast 实现

扩展一个函数 swizzleSayHello()

extension SayHello {
    class func swizzleSayHello() {
        let originalSlector = #selector(SayHello.sayHello(_:))
        let originalMethod = class_getInstanceMethod(SayHello.self, originalSlector)!
        let originalImplementation = method_getImplementation(originalMethod)
        //id (*IMP)(id, SEL, ...)
        typealias originalClosureType = @convention(c) (AnyObject,Selector,String) -> Void
        let originalClosure:originalClosureType = unsafeBitCast(originalImplementation, to: originalClosureType.self)
        let newBlock:@convention(block)(AnyObject,String) -> Void = {
            obj, name in
            var name__ = name
            name__ = name + ",nice to meet you"
            return originalClosure(obj,originalSlector,name__)
        }
            //Its signature should be: method_return_type ^(id self, method_args...).
        let newImplementation = imp_implementationWithBlock(unsafeBitCast(newBlock, to: AnyObject.self))
        print(newImplementation)
        method_setImplementation(originalMethod, newImplementation)
    }
}

调用函数

1 2	SayHello.swizzleSayHello() SayHello().sayHello("Jack")

打印：

1	Hello Jack,nice to meet you!

方案二：用method_exchangeImplementations实现

同样扩展一个函数 swizzleSayHello() 以及实现一个目标函数swizzledSayHello(_ name: String)

extension SayHello {
    //Argument of '#selector' refers to instance method 'swizzleSayHello' that is not exposed to Objective-C
    //Add '@objc' to expose this instance method to Objective-C
    //@objc 表示 暴露 方法给 oc 调用
     @objc private func swizzledSayHello(_ name: String) -> Void {
        var name__ = name
        name__ = name + ",nice to meet you"
        return swizzledSayHello(name__)
    }
    class func swizzledSayHello() {
        
        let originalSelctor = #selector(SayHello.sayHello(_:))
        let swizzledSelector = #selector(SayHello.swizzledSayHello(_:))
        swizzleMethod(for: SayHello.self, originalSelector: originalSelctor, swizzledSelector: swizzledSelector)
    }
    private class func swizzleMethod(for aClass: AnyClass, originalSelector: Selector,swizzledSelector: Selector) {
        let originalMethod = class_getInstanceMethod(aClass, originalSelector)
        let swizzledMethod = class_getInstanceMethod(aClass, swizzledSelector)
        let didAddMethod = class_addMethod(aClass, originalSelector, method_getImplementation(swizzledMethod!), method_getTypeEncoding(swizzledMethod!))
        if didAddMethod {
            class_replaceMethod(aClass, swizzledSelector, method_getImplementation(originalMethod!), method_getTypeEncoding(originalMethod!))
        } else {
            method_exchangeImplementations(originalMethod!, swizzledMethod!)
        }
    }
}